In the key of C the Dm7 is called the IIm7 and the II7 is a D7. Depending on the chord structure it may not always be the IIm7. It most certainly could be a II7.

When using the roman numeral system, the II chord needs to be addressed as either IIm7, IIm7#9, IIm7b5 etc so on and so forth. The D7 should be addressed as II7. A D7b9 would look like II7b9. It could also be IIM7. The capitol "M" denotes a D Maj7th.

We used the number system in 1959 in the group I worked with. The bass player introduced the system to me. Way back then with a song request that I may not have been familiar with, while playing live, that the rest of the band knew and wanted to play, I could play a solo on tunes I'd never heard before. All I needed was for the Bass player to whisper the chords to me as the song progressed during my solo. Such as: I, IV, II7, V7 in my ear.

Many musicians can do this. I’m not a genius. I’m a Cajun who cheated his way through high school. No I’m not proud of that statement. OK.

If you figure the number system out, all you need to know is what key you are in. It works in any key. Think about it for a few weeks and it becomes clearer. Some of you I know, know and understand this already. It’s not for you, it’s for the ones who don’t know this system that I‘m trying to explain this to.

I‘m not trying to show off. In jazz it’s a must. Bunky Green, Charlie Bird Parker and those guys probably knew this stuff before I was born. Well maybe not before I was born1, but definitely before a young whippersnapper like Dave was born. Got em again. Man! I just love it!

In the old days if it was a Dm7 in the key of C, the bass player would (literally) say II minor seven. If it is a II7 he just says II seven. This is simpler than using the letter names of a chord after you learn the system. Of course, if you aren’t familiar with the number system, then it is going to be like a foreign language to you

Notice for the II7 he didn’t need to say “D major chord with a dominant 7th. He just simply said II7. Musicians who use this system understand this. Once you get the concept of this system you really get to appreciate it.

All of this is insignificant in a one-man-band which is what this forum is about and I’m elated to be here. But, if you play with other musicians all the time, it would be beneficial for you guys to figure this out. It’s like short hand I guess if you were a writer. Eh Gary?

I know I’m boring most of you, however, I hope maybe I’ve helped a curious mind.